SELKIRK CORP 103400 3-Inch  Support Assembly 3" inch, 3RV-GVS, Support Assembly, Round, Type B, Gas Vent Support Plate should be installed above the joist Support plate also serves as a firestop Will support a maximum of 60' of gas vent RV line of MetalBest round Type B gas vent pipe and fittings is designed for use on approved gas appliances